Except fast is an adjective when describing food... Just because the phrase is used colloquially to describe a specific group of restaurants doesn't change the

Except fast is an adjective when describing food... Just because the phrase is used colloquially to describe a specific group of restaurants doesn't change the fact the phrase "fast food" is comprised of two words, one being a noun; the other an adjective. Fast greasy food. Fast disgusting food. Fast food is not a word. It is a phrase. Words don't have spaces.